Entangled-type carbon nanotubes and method for preparing the same

ABSTRACT

The present invention relates to entangled-type carbon nanotubes which have a bulk density of 31 kg/m 3  to 85 kg/m 3  and a ratio of tapped bulk density to bulk density of 1.37 to 2.05, and a method for preparing the entangled-type carbon nanotubes.

The present application is Divisional of U.S. patent application Ser.No. 16/628,555, filed Jan. 3, 2020 and published on Jul. 30, 2020 as US2020/0239315, which is a National Phase entry pursuant to 35 U.S.C. §371 of International Application No. PCT/KR2018/014738 filed on Nov. 27,2018, and claims priority to and the benefit of Korean PatentApplication Nos. 10-2017-0179768 filed on Dec. 26, 2017 and10-2018-0146925 filed on Nov. 26, 2018, the disclosures of which areincorporated herein by reference in their entirety.

FIELD

The present invention relates to entangled-type carbon nanotubes and amethod for preparing the entangled-type carbon nanotubes, and moreparticularly, to entangled-type carbon nanotubes exhibiting enhanceddispersibility and conductivity by adjusting a ratio of tapped bulkdensity to bulk density, and a method for preparing the entangled-typecarbon nanotubes.

BACKGROUND

Carbon nanotubes, which are a type of fine carbon fibers, are a tubularform of carbon having an average diameter of 1 μm or less, and areexpected to be applied to various fields due to their high conductivity,tensile strength, and heat resistance derived from their uniquestructures. However, despite the availability of such carbon nanotubes,carbon nanotubes have limitations in usage due to their low solubilityand dispersibility. Thus, a conductive material dispersion prepared bypre-dispersing carbon nanotubes in a dispersion medium has been used.However, carbon nanotubes are unable to form a stably dispersed state ina dispersion medium and are agglomerated with each other due to strongVan der Waals interactions.

To address these problems, various attempts have been made.Specifically, methods of dispersing carbon nanotubes in a dispersionmedium through mechanical dispersion treatment such as ultrasonictreatment, or the like have been proposed. However, when these methodsare used, excellent dispersibility is obtained during irradiation ofultrasonic waves, but carbon nanotubes start to agglomerate with eachother when ultrasonic irradiation is completed. In addition, methods ofstably dispersing carbon nanotubes using a variety of dispersants havebeen proposed. However, these methods also have a problem such asdifficulty in handling due to an increase in viscosity when carbonnanotubes are dispersed in a dispersion medium at a high concentration.

Therefore, there is a need to develop carbon nanotubes with enhanceddispersibility without a reduction in conductivity.

SUMMARY

An object of the present invention is to provide entangled-type carbonnanotubes having excellent dispersibility and excellent conductivity,and a method for preparing the carbon nanotubes.

To address the above-described technical problem, the present inventionprovides entangled-type carbon nanotubes which have a bulk density of 31kg/m³ to 85 kg/m³ and satisfy the following Equation 1:

1.37≤X/Y≤2.05  <Equation 1>

wherein, in Equation 1, X denotes a tapped bulk density (units: kg/m³)of the entangled-type carbon nanotubes, and

Y denotes a bulk density (units: kg/m³) of the entangled-type carbonnanotubes.

The present invention also provides a method for preparingentangled-type carbon nanotubes, comprising: mixing an organic acid anda vanadium precursor in a molar ratio of 1:0.0463 to 1:0.0875 to preparea mixture; mixing the mixture and a cobalt precursor to prepare acatalyst precursor; performing first heat treatment on aluminumhydroxide to prepare a support; supporting the catalyst precursor on thesupport and performing second heat treatment on the resulting support toprepare a supported catalyst; and reacting the supported catalyst with acarbon-based compound.

Entangled-type carbon nanotubes according to the present invention haveexcellent conductivity and excellent dispersibility, and thus can beincluded in a carbon nanotube dispersion at a high concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope image showing surfaces ofentangled-type carbon nanotubes of Example 3 at a magnification of 400×.

FIG. 2 is a scanning electron microscope image showing surfaces of theentangled-type carbon nanotubes of Example 3 at a magnification of1,000×.

FIG. 3 is a scanning electron microscope image showing surfaces ofentangled-type carbon nanotubes of Example 4 at a magnification of 400×.

FIG. 4 is a scanning electron microscope image showing surfaces of theentangled-type carbon nanotubes of Example 4 at a magnification of1,000×.

DETAILED DESCRIPTION

Hereinafter, the present invention will be described in more detail toaid in understanding of the present invention.

The terms or words used in the present specification and claims shouldnot be construed as being limited to ordinary or dictionary meanings andshould be construed as meanings and concepts consistent with the spiritof the present invention based on a principle that an inventor canappropriately define concepts of terms to explain his/her invention inthe best way.

In the present invention, carbon nanotubes refer to pristine carbonnanotubes that have not undergone separate processing.

In the present invention, entangled-type carbon nanotubes refer to asecondary structural form in which a plurality of carbon nanotube unitsare entangled with each other without a certain form such as a bundle ora rope.

In the present invention, bundle-type carbon nanotubes refer to thearrangement of a plurality of carbon nanotube units such thatlongitudinal axes of the units are parallel to each other insubstantially the same orientation, or a secondary structural formtwisted or entangled into a bundle or rope form after the arrangement.

In the present invention, the carbon nanotube unit has a graphite sheetin the form of a cylinder having a nano-sized diameter, and has a sp²bonding structure. In this case, the graphite sheet may exhibitcharacteristics of a conductor or a semiconductor according to the woundangle and structure. The carbon nanotube units may be classified intosingle-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes(DWCNTs), and multi-walled carbon nanotubes (MWCNTs) according to thenumber of bonds forming the walls, and the thinner the walls, the lowerthe resistance.

In the present invention, the bulk density of carbon nanotubes may bemeasured in accordance with ASTM B329, particularly ASTM B329-06. Inaddition, the bulk density may be measured using a Scott volumeter(Version USP 616).

In the present invention, the bulk density of the carbon nanotubes maybe measured in accordance with laboratory conditions, and substantiallythe same results as those based on the above-described rule may beobtained.

For the measurement of bulk density in a laboratory, a 5 ml cylinder(manufacturer: DURAN, material: glass) is placed on a scale, the scaleis calibrated to 0.5 ml of carbon nanotubes are added to the cylinder,the volume of the carbon nanotubes is measured by reading the scaleafter adjusting the eye level to the height of the carbon nanotubes, thecarbon nanotubes are weighed, and then the bulk density may becalculated using the following Equation:

Bulk density(kg/m ³)=weight(kg)of carbon nanotubes/volume(m ³)of carbonnanotubes

In the present invention, the tapped bulk density (TD) of carbonnanotubes may be measured in accordance with ASTM B527-06, particularlyusing TAP-2S available from LOGAN.

In the present invention, the tapped bulk density of carbon nanotubesmay be measured in accordance with laboratory conditions, and even inthe case of measurement in accordance with laboratory scales,substantially the same results as those based on the rule may beobtained.

For measurement of the tapped bulk density in a laboratory, a 5 mlcylinder (manufacturer: DURAN, material: glass) is placed on a scale,the scale is calibrated to 0, carbon nanotubes are added to thecylinder, the volume of the carbon nanotubes is measured by reading thescale after adjusting the eye level to the height of the carbonnanotubes, and then the carbon nanotubes are weighed. After tapping thecylinder lightly about 100 times on the floor, the volume of the carbonnanotubes is measured by reading the gradation of the cylinder, and thetapped bulk density may be calculated by the following Equation:

Tapped bulk density (kg/m³)=(weight (kg) of carbon nanotubes/volume (m³)of carbon nanotubes after tapping 100 times)

In the present invention, the specific surface area of carbon nanotubesmay be measured by a BET method, and may be calculated from the amountof nitrogen gas adsorbed at a liquid nitrogen temperature (770 K) using,for example, BELSORP-mini II manufactured by BEL Japan.

In the present invention, the average diameter and length of the carbonnanotube units may be measured using a field emission scanning electronmicroscope.

1. Entangled-Type Carbon Nanotubes

Entangled-type carbon nanotubes according to an embodiment of thepresent invention have a bulk density of 31 kg/m³ to 85 kg/m³ andsatisfy the following Equation 1:

1.37≤X/Y≤2.05  <Equation 1>

wherein, in Equation 1, X denotes a tapped bulk density (units: kg/m³)of the entangled-type carbon nanotubes, and

Y denotes a bulk density (units: kg/m³) of the entangled-type carbonnanotubes.

When the bulk density of the entangled-type carbon nanotubes is lessthan the above-described range, the characteristics of theentangled-type carbon nanotubes are unable to be realized, and thuscarbon nanotubes cannot be dispersed in a dispersion medium at a highconcentration when a carbon nanotube dispersion is prepared. When thebulk density of the entangled-type carbon nanotubes exceeds theabove-described range, a space between carbon nanotube unitsconstituting the entangled-type carbon nanotubes is so compact that thecarbon nanotubes are not easily untangled in a solvent. Thus, in aprocess of dispersing the entangled-type carbon nanotubes, the carbonnanotube units are more likely to be broken and consequently,conductivity may be reduced.

The entangled-type carbon nanotubes may have a bulk density ofpreferably 32 kg/m³ to 80 kg/m³, more preferably 32 kg/m³ to 68 kg/m³.When the bulk density of the entangled-type carbon nanotubes is withinthe above range, the carbon nanotubes may have sufficient particleproperties, and thus the carbon nanotubes may be slowly dispersed at ahigh concentration in a carbon nanotube dispersion preparation process.

Equation 1 is an index showing the morphology of the entangled-typecarbon nanotubes, and a value of Equation 1 is 1.37 to 2.05, preferably1.4 to 2.0, and more preferably 1.49 to 2.0. When the value of Equation1 is less than the above-described range, this means entangled-typecarbon nanotubes in which carbon nanotube units are very denselyentangled with each other, and thus it is difficult for the carbonnanotube units to be easily dispersed when a carbon nanotube dispersionis prepared. When the value of Equation 1 exceeds the above-describedrange, this means a wide interval between carbon nanotube units due tointeractions therebetween, and thus it is difficult for the carbonnanotube units to be dispersed at a high concentration when a carbonnanotube dispersion is prepared.

The entangled-type carbon nanotubes, which have a bulk density withinthe above-described range and satisfy the condition of Equation 1, mayhave sufficient particle properties like existing entangled-type carbonnanotubes and have a loose structure between carbon nanotube units likebundle-type carbon nanotubes. That is, the entangled-type carbonnanotubes may be of an entangled type, but may also have somecharacteristics of bundle-type carbon nanotubes. Accordingly, theentangled-type carbon nanotubes may be slowly dispersed and thusdispersed at a high concentration in a carbon nanotube dispersionpreparation process. In addition, since an interval between the carbonnanotube units is not compact, the carbon nanotube units may be moreeasily untangled than existing entangled-type carbon nanotube units,when dispersed in a dispersion medium. Thus, breakage of the carbonnanotube units may be reduced in the dispersion process, andconsequently, relatively long carbon nanotube units are present in adispersion medium. Accordingly, the carbon nanotube dispersion mayexhibit further enhanced conductivity.

The entangled-type carbon nanotubes may have a tapped bulk density ofpreferably 63 kg/m³ to 116 kg/m³, more preferably 65 kg/m³ to 102 kg/m³.When the tapped bulk density of the entangled-type carbon nanotubes iswithin the above-described range, the carbon nanotube units may beeasily untangled in a dispersion medium due to a less compact intervalbetween the carbon nanotube units than that of existing entangled-typecarbon nanotube units, and thus the breakage of the carbon nanotubeunits may be reduced in the dispersion process, and consequently,relatively long carbon nanotube units may be present in a dispersionmedium. Accordingly, the carbon nanotubes may exhibit further enhancedconductivity.

The entangled-type carbon nanotubes may have a BET specific surface areaof 100 m²/g to 300 m²/g, preferably 150 m²/g to 280 m²/g, and morepreferably 170 m²/g to 250 m²/g. When the BET specific surface area ofthe entangled-type carbon nanotubes is within the above-described range,the entangled-type carbon nanotubes have excellent powder resistivityand are suitable for use in high-concentration dispersions.

The entangled-type carbon nanotubes may have a powder resistance of0.0171 Ω·cm or less and a maximum dispersion concentration of 3.3 wt %or more, preferably a powder resistance of 0.0170 Ω·cm or less and amaximum dispersion concentration of 3.4 wt % or more, and morepreferably a powder resistance of 0.0168 Ω·cm or less and a maximumdispersion concentration of 3.5 wt % or more. When the powder resistanceand maximum dispersion concentration of the entangled-type carbonnanotubes are within the above-described ranges, highly conductiveentangled-type carbon nanotubes may be included in a carbon nanotubedispersion at a high concentration, and thus may be more suitable forused as a conductive material.

In this regard, the maximum dispersion concentration of theentangled-type carbon nanotubes may be a measured maximum amount ofcarbon nanotubes dispersible in a carbon nanotube dispersion prepared byadding the carbon nanotubes to N-methylpyrrolidone little by little. Inaddition, the powder resistance of the entangled-type carbon nanotubesmay be calculated from surface current and voltage measured using 4probes through Loresta-GX (Product Name, Manufacturer: MITSUBISHICHEMICAL ANALYTECH) after filling an insulating mold with entangled-typecarbon nanotubes so as to have a density of 1 g/cc and applying apressure thereto.

The carbon nanotube units may have an average diameter of preferably 30nm or less, more preferably 10 nm to 30 nm. When the average diameter ofthe carbon nanotube units is within the above-described range,dispersibility and conductivity may be enhanced.

The carbon nanotube units may have an average length of preferably 0.5μm to 200 μm, more preferably 10 μm to 60 μm. When the average length ofthe carbon nanotube units is within the above-described range, carbonnanotube units exhibit excellent electrical conductivity and excellentstrength, and are stable both at room temperature and a hightemperature.

The carbon nanotube units may have an aspect ratio of preferably 5 to50,000, more preferably 10 to 20,000, wherein the aspect ratio isdefined as a ratio of the length (the length of a major axis passingthrough the center of the unit) of the carbon nanotube unit to thediameter (the length of a minor axis passing through the center of theunit and perpendicular to the major axis) of the carbon nanotube unit.

The average diameter and average length of the carbon nanotube units maybe measured using a field emission scanning electrode microscope.

The carbon nanotube units have an interlayer distance (d₀₀₂) obtained byX-ray diffraction of a carbon crystal of 0.335 nm to 0.342 nm, maysatisfy the following condition: interlayer distance(d₀₀₂)<0.3448-0.0028 (log φ) wherein φ is an average diameter of thecarbon nanotube units, and may have a thickness (Lc) in a C-axisdirection of the crystal of 40 nm or less.

The interlayer distance (d₀₀₂) may preferably be less than 0.3444-0.0028(log φ), more preferably less than 0.3441-0.0028 (log φ).

When the interlayer distance (d₀₀₂) is within the above-described range,the carbon nanotube units may exhibit enhanced crystallinity, andentangled-type carbon nanotubes including the carbon nanotube units mayexhibit further enhanced conductivity.

2. Method for Preparing Entangled-Type Carbon Nanotubes

Entangled-type carbon nanotubes according to an embodiment of thepresent invention are prepared using a method comprising: 1) mixing anorganic acid and a vanadium precursor in a molar ratio of 1:0.0463 to1:0.0875 to prepare a mixture; 2) mixing the mixture and a cobaltprecursor to prepare a catalyst precursor; 3) performing first heattreatment on aluminum hydroxide to prepare a support; 4) supporting thecatalyst precursor on the support and performing second heat treatmenton the resulting support to prepare a supported catalyst; and 5)reacting the supported catalyst with a carbon-based compound.

Hereinafter, each process of the method for preparing entangled-typecarbon nanotubes according to an embodiment of the present inventionwill be described in more detail.

1) Preparation of Mixture

First, an organic acid and a vanadium precursor are mixed in a molarratio of 1:0.0463 to 1:0.0875 to prepare a mixture.

The organic acid and the vanadium precursor may be mixed in a molarratio of preferably 1:0.047 to 1:0.086, more preferably 1:0.0475 to1:0.077. When the mixing molar ratio is within the above-describedrange, entangled-type carbon nanotubes that are dispersible in aconductive material dispersion at a high concentration and have a lowbulk density and a low tapped bulk density may be manufactured.

When the molar ratio of the organic acid to the vanadium precursor isless than the above-described range, a particle size distribution ofcatalyst particles decreases. When the molar ratio of the organic acidto the vanadium precursor exceeds the above-described range, bundle-typecarbon nanotubes in addition to the entangled-type carbon nanotubes aremanufactured.

The organic acid may be one or more selected from the group consistingof citric acid, tartaric acid, fumaric acid, malic acid, acetic acid,butyric acid, palmitic acid, and oxalic acid, and among these organicacids, citric acid is preferably used.

The vanadium precursor may be a salt of a vanadium compound, andpreferably may be one or more selected from the group consisting ofNH₄VO₃, NaVO₃, V₂O₅, and V(C₅H₇O₂)₃, and among them, NH₄VO₃ is morepreferably used.

2) Preparation of Catalyst Precursor

Subsequently, the mixture is mixed with a cobalt precursor to prepare acatalyst precursor.

The mixture and the cobalt precursor may be mixed such that a molarratio of vanadium and cobalt becomes 1:1 to 1:100, preferably 1:5 to1:20. When the molar ratio is within the above-described range, yield isincreased.

The cobalt precursor may be a salt of a cobalt compound, and preferablymay be one or more selected from the group consisting of Co(NO₃)₂·6H₂O,CoCl₂·6H₂O, Co₂(CO)₈, and [Co₂(CO)₆(t−BuC═CH)], and among them,Co(NO₃)₂·6H₂O is more preferably used.

The mixture and the cobalt precursor, i.e., the organic acid, thevanadium precursor, and the cobalt precursor, may be used in the form ofa solution dissolved in a solvent, and the solvent may be one or moreselected from the group consisting of water, methanol, and ethanol, andamong them, water is preferably used.

The concentration of the citric acid, the vanadium precursor, and thecobalt precursor in the solution may range from 0.1 g/ml to 3 g/ml,preferably 0.5 g/ml to 2 g/ml, and more preferably 0.7 g/ml to 1.5 g/ml.

3) Preparation of Support

Subsequently, aluminum hydroxide (Al(OH)₃) is subjected to first heattreatment to prepare a support.

The aluminum hydroxide may be pretreated before the first heattreatment.

The pretreatment process may be performed at a temperature of 50° C. to150° C. for 1 hour to 24 hours. When the pretreatment process isperformed, the residual solvent or impurities that may be present on asurface of the aluminum hydroxide may be removed.

The aluminum hydroxide may have an average particle diameter of 20 μm to200 μm, a porosity of 0.1 cm³/g to 1.0 cm³/g, and a specific surfacearea of less than 1 m²/g.

The first heat treatment process may be performed at a temperature of250° C. to 500° C., preferably 400° C. to 500° C. In addition, the firstheat treatment process may be performed in an air atmosphere.

Under the above-described conditions, a support including AlO(OH) andAl(OH)₃, which are obtained by conversion of aluminum hydroxide, inamounts of 30 wt % or more and 70 wt % or less, respectively,particularly 40 wt % or more and 60 wt % or less, respectively, and notincluding Al₂O₃ may be prepared.

The support may further include a metal oxide such as ZrO₂, MgO, SiO₂,or the like.

The shape of the support is not particularly limited, but the supportmay have a spherical shape or a potato shape. In addition, the supportmay have a porous structure, a molecular sieve structure, a honeycombstructure, or the like so as to have a relatively large surface area perunit mass or unit volume.

4) Preparation of Supported Catalyst

Subsequently, the catalyst precursor is supported on the support, andthen subjected to second heat treatment, thereby preparing a supportedcatalyst.

The supporting process may be performed by uniformly mixing the supportand the catalyst precursor, and then aging the resulting mixture for acertain period of time. The mixing process may be performed by rotationor stirring, particularly at a temperature of 45° C. to 80° C. The agingprocess may be performed for 3 minutes to 60 minutes.

The catalyst precursor may be further subjected to drying after beingsupported on the support.

The drying process may be performed at a temperature of 60° C. to 200°C. for 4 hours to 16 hours.

The second heat treatment process may be performed in an air atmospherefor 1 hour to 6 hours. The second heat treatment process may beperformed at a temperature of preferably 700° C. to 800° C. When thesecond heat treatment temperature is within the above-described range, asupported catalyst, in which the catalyst precursor is present in astate of being coated on a surface and fine pores of the support, isprepared. In addition, entangled-type carbon nanotubes, which are afinal product manufactured using the supported catalyst, have a bulkdensity within the above-described range and satisfy the condition ofEquation 1.

5) Reaction between Supported Catalyst and Carbon-based Compound

Subsequently, the supported catalyst is reacted with a carbon-basedcompound.

The reaction between the supported catalyst and a carbon-based compoundmay be carried out by a chemical vapor synthesis method.

Specifically, the reaction may be performed by feeding the supportedcatalyst into a horizontal fixed bed reactor or a fluidized bed reactor,and injecting the carbon-based compound in a gaseous state (hereinafter,referred to as “gas-phase”), or the gas-phase carbon-based compound anda mixed gas of a reducing gas (e.g., hydrogen, or the like) and acarrier gas (e.g., nitrogen, or the like) into the reactor at atemperature of a thermal decomposition temperature or more of thecarbon-based compound and a melting point or less of a catalyst of thesupported catalyst, thereby growing carbon nanotubes using a chemicalvapor synthesis method through decomposition of the gas-phasecarbon-based compound. Carbon nanotubes manufactured by the chemicalvapor synthesis method have a crystal growth direction almost parallelto a tube axis and have high crystallinity of a graphite structure in atube longitudinal direction. As a result, carbon nanotube units have asmall diameter, high electrical conductivity, and high strength.

In addition, the entangled-type carbon nanotubes may be manufactured ata temperature of particularly 500° C. to 800° C., more particularly 550°C. to 750° C. With the above-described reaction temperature range, theweight of the entangled-type carbon nanotubes may be reduced while thegeneration of non-crystalline carbon is minimized and a bulk size ofproduced carbon nanotubes is maintained, and thus dispersibilityaccording to a decrease in bulk density may be further enhanced. As aheat source for the heat treatment processes, induction heating,radiation heat, a laser, IR, microwaves, plasma, surface plasmonheating, or the like may be used.

In addition, carbon may be supplied as the carbon-based compound, andthe carbon-based compound is not particularly limited as long as it canbe present in a gaseous state at a temperature of 300° C. or more.

The carbon-based compound may be a carbon-based compound having 6 carbonatoms, and more particularly may be one or more selected from the groupconsisting of carbon monoxide, methane, ethane, ethylene, ethanol,acetylene, propane, propylene, butane, butadiene, pentane, pentene,cyclopentadiene, hexane, cyclohexane, benzene, and toluene.

In the manufacturing method of the present invention, a removal processfor removing metal catalyst-derived metal impurities remaining in theentangled-type carbon nanotubes may be optionally performed. In thisregard, the metal impurities removal process may be performed accordingto a general method such as washing, acid treatment, or the like.

EXAMPLES

Hereinafter, embodiments of the present invention will be described indetail in such a way that the invention may be carried out without unduedifficulty by one of ordinary skill in the art to which the presentinvention pertains. However, the present invention may be embodied inmany different forms and the following examples are not intended tolimit the scope of the present invention.

<Preparation of Entangled-Type Carbon Nanotubes>

Example 1

Aluminum hydroxide (Al(OH)₃) as an aluminum-based support precursor wassubjected to first heat treatment in an air atmosphere at 450° C. for 4hours, thereby preparing an aluminum-based support including 40 wt % ormore of AlO(OH).

Separately, citric acid and NH₄VO₃ were added into water in a molarratio of 1:0.0475 to prepare an aqueous NH₄VO₃ solution. Co(N03)₂·6H₂Oand the aqueous NH₄VO₃ solution were mixed such that a molar ratio of Coto V was 10:1, thereby preparing an aqueous catalyst precursor solution,which is a clear aqueous solution.

The support and the aqueous catalyst precursor solution were mixed suchthat the respective amounts of Co and V in the aqueous catalystprecursor solution were 23 moles and 2.3 moles with respect to 100 molesof Al in the support.

The aqueous catalyst precursor solution was supported on the support ina 60° C. thermostatic bath for 5 minutes, and then dried in an airatmosphere at 120° C. for 12 hours. Subsequently, the resulting supportwas subjected to second heat treatment in an air atmosphere at 720° C.for 4 hours, thereby preparing a supported catalyst.

0.1 g of the supported catalyst was placed in the center of a quartztube having an inner diameter of 55 mm located in a fixed bed reactor.The inside of the fixed bed reactor was heated to 650° C. in a nitrogenatmosphere and maintained, followed by synthesis for 60 minutes whileflowing a mixture of nitrogen, ethylene gas, and hydrogen gas in avolume ratio of 1:1:1 at a rate of 0.3 t/min, thereby obtainingentangled-type carbon nanotubes.

Example 2

Entangled-type carbon nanotubes were prepared in the same manner as inExample 1, except that citric acid and NH₄VO₃ were added to water in amolar ratio of 1:0.05 and dissolved to prepare an aqueous NH₄VO₃solution.

Example 3

Entangled-type carbon nanotubes were prepared in the same manner as inExample 1, except that citric acid and NH₄VO₃ were added to water in amolar ratio of 1:0.072 and dissolved to prepare an aqueous NH₄VO₃solution.

Example 4

Entangled-type carbon nanotubes were prepared in the same manner as inExample 1, except that citric acid and NH₄VO₃ were added to water in amolar ratio of 1:0.082 and dissolved to prepare an aqueous NH₄VO₃solution.

Example 5

Entangled-type carbon nanotubes were prepared in the same manner as inExample 1, except that citric acid and NH₄VO₃ were added to water in amolar ratio of 1:0.085 and dissolved to prepare an aqueous NH₄VO₃solution.

Comparative Example 1

Entangled-type carbon nanotubes were prepared in the same manner as inExample 1, except that citric acid and NH₄VO₃ were added to water in amolar ratio of 1:0.045 and dissolved to prepare an aqueous NH₄VO₃solution.

Comparative Example 2

Entangled-type carbon nanotubes were prepared in the same manner as inExample 1, except that citric acid and NH₄VO₃ were added to water in amolar ratio of 1:0.09 and dissolved to prepare an aqueous NH₄VO₃solution.

Comparative Example 3

Carbon nanotubes were prepared in the same manner as in Example 1,except that citric acid and NH₄VO₃ were added to water in a molar ratioof 1:5.8 and dissolved to prepare an aqueous NH₄VO₃ solution, but themanufactured carbon nanotubes were a bundle type.

Comparative Example 4

Entangled-type carbon nanotubes (manufacturer: Bayer, Product Name:C150P) were used.

Comparative Example 5

Entangled-type carbon nanotubes (manufacturer: LG Chem, Ltd.) were used.

Experimental Example 1

The entangled-type carbon nanotubes prepared according to Examples 3 and4 were photographed using a scanning electrode microscope (SEM), and theresults of Example 3 are shown in FIGS. 1 and 2 , and the results ofExample 4 are shown in FIGS. 3 and 4 .

In this regard, FIGS. 1 and 3 are enlarged SEM images of surfaces of theentangled-type carbon nanotubes at a magnification of 400×. FIGS. 2 and4 are enlarged SEM images of surfaces of the entangled-type carbonnanotubes at a magnification of 1,000×.

Referring to FIGS. 1 to 4 , it was confirmed that the carbon nanotubesof Examples 3 and 4 were an entangled-type.

Experimental Example 2

Physical properties of the carbon nanotubes of the examples and thecomparative examples were measured using the following methods, and theresults thereof are shown in Tables 1 and 2 below.

(1) Production yield: {(total weight of prepared carbonnanotubes)−(total weight of supported catalyst used)}/(total weight ofsupported catalyst used)

(2) Bulk density: A 5 ml cylinder (manufacturer: DURAN, material:glass), a weight of which was known, was filled with carbon nanotubes,the cylinder was weighed, and then the bulk density was calculatedaccording to the following Equation.

Bulk density(kg/m ³)=weight(kg)of carbon nanotubes/volume(m ³)of carbonnanotubes

(3) Tapped bulk density: The tapped bulk density was measured usingTAP-2S manufactured by LOGAN in accordance with ASTM B527-06.

(4) Specific surface area (m²/g): Specific surface area may becalculated from the amount of nitrogen gas adsorbed at a liquid nitrogentemperature (77 K), using BELSORP-mini II manufactured by BEL Japan.

(5) Secondary structural form: It was confirmed through an SEM.

(6) Powder resistance (ohm cm @ 1 g/cc): An insulating mold was filledwith carbon nanotubes so as to have a density of 1 g/cc, a pressure wasapplied thereto, surface current and voltage were measured using 4probes through Loresta-GX(Product Name, manufacturer: MITSUBISHICHEMICAL ANALYTECH), and a powder resistance was calculated therefrom.

(7) Maximum dispersion concentration (wt %): A carbon nanotubedispersion was prepared by adding carbon nanotubes toN-methylpyrrolidone little by little. Then, a maximum dispersionconcentration of the carbon nanotubes dispersible in the carbon nanotubedispersion was measured.

TABLE 1 Classification Example 1 Example 2 Example 3 Example 4 Example 5Molar ratio of citric 1:0.0475 1:0.05 1:0.072 1:0.082 1:0.085 acid toNH₄VO₃ Secondary structural Entangled- Entangled- Entangled- Entangled-Entangled- form type type type type type Production yield 11 11 18.521.8 22.7 Bulk density 32 36 61 76 80 (kg/m³) Tapped bulk density 64 6593 111 112 (kg/m³) Tapped bulk 2.0 1.8 1.52 1.46 1.41 density/bulkdensity Powder resistance 0.0162 0.0161 0.0166 0.0170 0.0171 (ohm · cm @1 g/cc) Maximum dispersion 3.5 3.5 3.5 3.5 3.5 concentration (wt %)

TABLE 2 Comparative Comparative Comparative Comparative ComparativeClassification Example 1 Example 2 Example 3 Example 4 Example 5 Molarratio of citric 1:0.045 1:0.09 1:5.8 — — acid to NH₄VO₃ Secondarystructural Entangled- Entangled- Bundle type Entangled- Entangled- formtype type type type Production yield 10 24 19.2 20 44 Bulk density 29 9045 150 172 (kg/m³) Tapped bulk density 61 120 75 172 208 (kg/m³) Tappedbulk 2.1 1.33 1.66 1.17 1.21 density/bulk density Powder resistance0.0171 0.0181 0.0088 0.0180 0.0179 (ohm · cm @ 1 g/cc) Maximumdispersion 2.0 3.0 1.25 3.5 3.5 concentration (wt %)

Referring to Tables 1 and 2, the entangled-type carbon nanotubes ofExamples 1 to 5 prepared by adding citric acid and NH₄VO₃ in a molarratio of 1:0.0475 to 1:0.085 had a bulk density of 32 kg/m³ to 80 kg/m³and satisfied the condition of Equation 1. In addition, theentangled-type carbon nanotubes of Examples 1 to 5 exhibited a lowpowder resistance and a high maximum dispersion concentration, and thusit was anticipated that the entangled-type carbon nanotubes weresuitable for use in a conductive material dispersion due to excellentconductivity and inclusion in a dispersion at a high concentration.

Meanwhile, it was confirmed that the entangled-type carbon nanotubes ofComparative Example 1 manufactured by adding citric acid and NH₄VO₃ in amolar ratio of 1:0.045 had a lower bulk density than that of the case ofthe present invention and were unable to satisfy the condition ofEquation 1. In addition, although the entangled-type carbon nanotubes ofComparative Example 1 had a low powder resistance, they could not beincluded in a dispersion at a high concentration, from which it wasanticipated that the entangled-type carbon nanotubes were not suitablefor use in a conductive material dispersion.

In addition, it was confirmed that the entangled-type carbon nanotubesof Comparative Example 2 manufactured by adding citric acid and NH₄VO₃in a molar ratio of 1:0.09 had a higher bulk density than that of thecase of the present invention and were unable to satisfy the conditionof Equation 1. The entangled-type carbon nanotubes of ComparativeExample 2 had a high powder resistance and could not be included in adispersion at a high concentration, and thus were anticipated to be notsuitable for use in a conductive material dispersion.

In addition, it was confirmed that the carbon nanotubes of ComparativeExample 3 manufactured by adding citric acid and NH₄VO₃ in a molar ratioof 1:5.8 were a bundle type and did not satisfy the condition ofEquation 1. In addition, although the bundle-type carbon nanotubes ofComparative Example 3 had a low powder resistance, they could not beincluded in a dispersion at a high concentration, and thus it wasanticipated that they were not suitable for use in a conductive materialdispersion.

It was also confirmed that the entangled-type carbon nanotubes ofComparative Examples 4 and 5, which are commercially available carbonnanotubes, had an excessively high bulk density and were also unable tosatisfy the condition of Equation 1. In addition, the entangled-typecarbon nanotubes of Comparative Examples 4 and 5 were anticipated to benot suitable for use in a conductive material dispersion due to theirhigh powder resistances.

1. A method for preparing entangled-type carbon nanotubes, comprising:mixing an organic acid and a vanadium precursor in a molar ratio of1:0.0463 to 1:0.0875 to prepare a mixture; mixing the mixture and acobalt precursor to prepare a catalyst precursor; performing first heattreatment on aluminum hydroxide to prepare a support; supporting thecatalyst precursor on the support and performing second heat treatmenton the resulting support to prepare a supported catalyst; and reactingthe supported catalyst with a carbon-based compound.
 2. The method ofclaim 1, wherein the preparation of the mixture comprises mixing anorganic acid and a vanadium precursor in a molar ratio of 1:0.047 to1:0.086.
 3. The method of claim 1, wherein the preparation of thecatalyst precursor comprises mixing the mixture and a cobalt precursorsuch that a molar ratio of vanadium and cobalt is in a range of 1:1 to1:100.
 4. The method of claim 1, wherein the organic acid comprises oneor more selected from the group consisting of citric acid, tartaricacid, fumaric acid, malic acid, acetic acid, butyric acid, palmiticacid, and oxalic acid.